Silicon wafers which are being processed in a semiconductor fabrication facility are typically held in cassettes when they are not undergoing processing. A standard cassette has been adopted according to specifications issued by the Semiconductor Equipment Manufacturers Institute (SEMI), and the standard cassette is used almost universally in the semiconductor industry. The cassettes containing silicon wafers must be transported from one production step to the next throughout a fabrication facility. Typically, these cassettes are carried inside plastic boxes. Once a box with a cassette arrives at a particular piece of process equipment, an operator opens the box, removes the cassette and places the cassette into the equipment.
The cassettes are positioned inside the plastic transport boxes with the "H" bar forward and the flat surfaces of the wafers oriented vertically. The force of gravity keeps the wafers seated in the cassette.
Most processing equipment requires that the cassettes be introduced into the equipment with the "H" bar down and the wafers oriented horizontally. This configuration allows the equipment to access the individual wafers for processing. Thus, when the cassette is placed into the equipment, it must be rotated after it has been removed from the transport box. This rotation is typically done by an operator grasping the cassette from the side with both hands and rotating his or her wrists 90.degree. while placing the cassette into the equipment. The repeated rotation of the filled cassettes, which typically weigh over seven pounds, has resulted in numerous repetitive use injuries, such as carpal tunnel syndrome, among equipment operators.
The processing equipment is increasingly being automated with robots. To keep the equipment safe for operators, it must be enclosed at all times. This requirement adds to the difficulty of cassette input and output. When an operator is inserting a cassette into or removing a cassette from the equipment, for example, a barrier must separate the operator and any potentially hazardous mechanisms in the equipment. The only alternative is to shut the equipment down at these times, but doing this detracts from the production rate of the equipment. To maximize the output of processed wafers, it is important that the equipment operate continuously. This means that the equipment should not be stopped in order to change cassettes.
Thus, there is a need for a cassette input/output unit which overcomes all of these problems, i.e., avoids the need for the operator to rotate the cassette when it is inserted into the equipment, provides a safety barrier at all times which prevents the operator from inadvertently making contact with robots or other hazardous mechanisms inside the equipment, and allows the equipment to be operated without interruption while cassettes are being exchanged.